1. Field of the Invention
This invention relates generally to the field of nanolaminates. More particularly, it relates to interface-defined nanolaminate materials and methods of making the same.
2. Description of the Related Art
In general, laminates are utilized in a variety of diverse fields and may be made with layers having a wide range of thickness. The terms “laminated materials” or “laminates” generally refer to materials that consist of many parallel, relatively thick layers (layer thickness>1 mm) of dissimilar materials. The properties of laminates are generally controlled by two factors: the properties of the material within the layers and the properties of the interfaces between the layers. When the number of layers is small (in this case a material usually referred to as “layered”), it is predominately the properties of the materials within the individual layers that define the properties of the whole laminate. However, as the number of layers increases, the properties of the interfaces between the dissimilar layers begin to impose an ever-increasing effect on the properties of the laminate. In some applications, it is the properties of the interfaces that are the determining factor in the performance of the whole laminate. For example, a reflecting insulator that consists of a number of metallic layers, each of which is an excellent conductor of heat and is separated from the next reflector by an air gap or vacuum, is, nevertheless, an excellent insulator because of the reflection and scattering of heat perpendicular to the metal/gas interfaces.
Laminates have many industrially useful properties. The properties of laminates are anisotropic, so they are often referred to as “two-dimensional materials” because their properties in the plane of the layers and perpendicular to that plane are drastically different. For example, heat conductivity in the crystal plane and perpendicular to the crystal planes of pyrolytic graphite can differ by three orders of magnitude; fracturing progresses easily within the individual glass planes in laminated glass but is quickly arrested in the direction perpendicular to the glass planes; electrical current propagates in but not perpendicular to the planes in metal/oxide laminates utilized in super-capacitors, etc. These anisotropic properties of laminates can be highly useful in impeding conduction of heat as well as fracture propagation, including damage caused by chemical attack. Regardless of the form of the propagating entity, laminate materials usually inhibit propagation of the energy or matter in the direction perpendicular to the layers, while dissipating this energy or matter principally along the surface (i.e. parallel to the plane of the layers) or within the interfaces.
From a conceptual point of view, the process of obstructing the energy propagation can be described by similar mathematics in all of these cases, as each interface constitutes a barrier that must be overcome by the incoming energy or matter in order to proceed through the material. Though each barrier may be small, the sheer number of them and their sequential nature ultimately overwhelms the incoming energy or matter and slows down its rate of flow through the material to a small fraction of the original value. To illustrate this point, consider, for example, that one barrier reflects or scatters only 0.01% (0.0001) of the incoming energy, letting 0.9999 through. 100,000 of these barriers placed at a distance of 100 nm apart would attenuate the flow of incoming energy to 1% of the starting value after a distance of only 1 cm. In most cases, the scattering at an interface is much higher than 0.01%. For the example of heat scattering at a metal/gas interface, the scattering is controlled by the emissivity and reflectivity of the metal surface, which can be above 50%. This high emissivity/reflectivity explains why only a few reflectors are often able to contain very high temperatures. However, even when the scattering coefficient is small, the sheer number of barriers gives tremendous power to the approach of laminated materials. This is one of the reasons why nanolaminates—laminates with individual layer thicknesses on the order of 1 to 999 nm and preferably from 1 to 100 nm—and superlattices, which are a subset of nanolaminates with coherent or atomically-coherent interfaces that produce distinctive superlattice peaks in X-ray diffraction, have attracted so much interest in the last several decades, both in research and industry.
In general, nanolaminates and superlattices are extremely finely-layered materials with individual layer thicknesses on the order of 1 to 10 nm. Traditionally, the term “superlattice” is used with layered materials that have coherent interfaces, i.e. the lattice planes are continuous from one phase to another across the interface. When the interfaces are incoherent, the material is usually referred to as “nanolayered.” In the present application, “nanolaminate” and “nanolaminate materials” will be used generally to refer to materials with individual layer thicknesses up to 999 nm. These nanolaminate materials have been found to have very intriguing and industrially useful properties. Important new properties such as superior hardness/toughness combination, excellent wear resistance, supermodulus effects, superconductivity, optical waveguide properties, and magnetic properties are active areas of research in conventional nanolaminates.
Presently, despite their attractive properties, conventional nanolaminates have some very serious drawbacks from the point of view of industrial and commercial applications. With the exception of some high tech uses that require very small samples such as reading heads in magnetic storage, nanolaminates are prohibitively expensive for most industrial applications. Laminates with individual layer thicknesses of ≧100 microns may be made using relatively simple approaches that sequentially place one layer on top of the other, such as through the use of the “doctor blade” approach with powder pastes, electrophoretic deposition, spraying from different nozzles, pre-formed tapes, or by dipping in and/or painting on wet slurries. A few industrial methods to make bulk nanolayered materials exist, such as the manufacture of nanolayered materials comprising exfoliated graphite, vermiculate, and mica-type thermal insulation, which produces a single-component i.e. single composition material with incoherent interfaces; however, the continuity of individual layers in such materials is limited by the size of flakes, which is normally below 1 cm. A multiple-extrusion step approach has also been utilized in the electronics industry to make nanometer-thick layers in Channeltron photo-multiplier tubes and superconductor wires; however, neither of these methods is applicable to fabrication of bulk quantities of two-dimensional nanolayered materials.
Manufacture of nanolaminates with coherent interfaces currently requires the use of techniques such as chemical vapor deposition, physical vapor deposition, atomic layer deposition, pulsed laser deposition, electro-deposition, as well as magnetically and electrostatically-assisted sputtering, in which layers are built-up one atom at a time. These manufacturing methods are generally very slow (1 to 5 μm/min) and require expensive equipment, very clean conditions, and high vacuum, as the nanolaminates are essentially built-up one atom at a time. Nanolaminates manufactured by these techniques usually have semi-coherent (with dislocations) or coherent (no dislocations) interfaces because the slow-rate atomic deposition technique produces atomic uniformity of the interface. However, the sample size of materials made in this way is limited, and the cost to make commercial products with these techniques is prohibitive, making scale-up of these methods to industrially meaningful proportions unrealistic.
Manufacture of nanolaminates comprising shapes with a high aspect ratio such as tubes or rods presents additional challenges. For example, fuel cells require tubes with a high aspect ratio and high conductivity through the walls, while tubes comprising thermally-insulating materials require low conduction i.e. high thermal resistance along the length of the tube. To obtain a suitable nanolaminate tube, a very thick nanolaminate comprising many layers is made so that the tube may be machined through the nanolaminate i.e. so that the layers are perpendicular to the axis of the resulting tube. Manufacturing a high aspect ratio, nanolaminate part with a sufficient number of layers is thus extremely expensive, time-consuming, and impractical using conventional methods.
In addition, while laminates and nanolaminates with coherent interfaces possess many useful properties such as high conductivity, enhanced bonding, and minimum distortion across the interface, coherency is frequently undesirable as materials with coherent interfaces are usually quite brittle and have low thermal stability. The desired degree of coherency at each interface depends on the application. As stated above, laminate materials usually inhibit propagation of the energy or matter in the direction perpendicular to the layers, while dissipating this energy or matter along the surface or within the individual interfaces. Thus, to inhibit the propagation of energy, such as thermal energy or crack propagation perpendicular to the interfaces, it is desirable to have an incoherent interface between the layers of the laminate because coherent interfaces do not effectively scatter the energy perpendicular to them. Furthermore, because coherency at the interfaces leads to poor thermal stability, most superlattices are unstable even at room temperatures and quickly interdiffuse, losing their nanoscopic properties at or just above ambient temperatures. For these reasons, even where the properties of coherent interfaces may be desired, some departure from coherency is often needed to reduce the high strain energy associated with the coherent interfaces, thus assuring stability at elevated temperatures and improved mechanical properties. Such departures from ideal coherency are often induced by raising the temperature of the substrate or the rate of deposition during the magnetron-assisted sputtering of nanolaminates.
Thus, a need exists for an industrially-scalable batch or continuous technique to produce large amounts of lower-cost nanolaminates. In addition, a need exists for a process to fabricate a low-porosity nanolaminate material in which each interface has a cross-sectional area of ≧0.1 square meter.